Several aspects of the immunobiology of invertebrates remain poorly known relative to our detailed understanding of the vertebrate immune response. Because many invertebrates serve as intermediate hosts or vectors of parasites of great medical or veterinary significance, it is imperative that we better understand how their immune systems recognize and respond to infection by parasites. Consequently, the long term goal of the proposed research is to elucidate the fundamental immunological mechanisms underlying susceptibility and resistance of freshwater snails to infection with larval digenetic trematodes. Using as a model system the snail Biomphalaria glabrata and the trematode Echinostoma paraensei, snail hemolymph lectins will be investigated as mediators of "non-self" recognition in interactions with trematode larvae. Prior work with other molluscs and with this particular model system provide strong justification for this effort. A comprehensive series of experiments is proposed to explore thoroughly the functional relevance of lectins to this host- parasite system. Using a combination of electrophoretic techniques and specific antibody probes, both circulating lectins and hemocyte-associated lectins (hemocytes are molluscan immune cells) will be studied to determine if lectin composition changes following exposure to trematode infection. Also, lectins produced by snails known to be susceptible or resistant to E. paraensei infection will be carefully examined for differences that may relate to immune competence. Lectins purified by affinity chromatography will be tested for their ability to mediate hemocyte attachment to (and killing of) trematode larvae. Experiments will also be undertaken to determine if hemocytes are responsible for synthesizing lectin molecules, and if secretory products released by trematode larvae can interfere with the production of, or anti-parasite functions of, lectins. The proposed work will identify snail-produced molecules with anti-parasite functions and will set the stage for future attempts to clone the corresponding genes. Potentially, such efforts will culminate in the development of innovative new approaches to trematode control.